Iron-aluminum alloys containing less than 84% by weight iron and an additive and process for preparing the same



June 4. 1968 s. CABANE ETAL 3,336,819

IRON-ALUMINUM ALLOYS CONTAINING LESS THAN 84 BY WEIGHT IRON AND ANADDITIVE AND PROCESS FOR PREPARING THE SAME Filed March 17, 1965 TIMEHOURS STAINLESS STEEL Fe-Al ALLOY ITO EY United States Patent 0 M 4Claims. ci. 75-124 ABSTRACT 9F THE DISCLGSURE Iron-aluminum alloys ofgreater than 16% by weight aluminum and less than 84% by Weight iron andcontaining from 0.4% to 4% by weight, preferably 1% to 4% by weight, ofan additive selected from the group consisting of zirconium, nobium,titanium, yttrium, the rare earths and boron and a process for preparingsaid alloys. Advantageously, the amount of iron ranges from 69% to 82%by weight, and the amount of aluminum from 18% to 31% -by weight.

The present application is a continuation-in-part of application Ser.No. 261,152, filed Feb. 26, 1963, now U.S. Patent 3,303,561. The presentapplication accordingly incorporates by reference the disclosure in saidU.S. Patent 3,303,561 and the same is to be read as being a part of thepresent disclosure.

The present invention relates to a process for the preparation of aniron-aluminum alloy and, by way of new industrial products, the alloysobtained as a result of the application of said process.

Accordingly, it is an object of the present invention to provide aprocess for the manufacture of an alloy containing principally iron andaluminum of which the proportion of aluminum may be increasedconsiderably without the attendant difficulties encountered heretofore.

Another object of the present invention resides in the provision of aprocess for producing an iron-aluminum alloy in which the aluminumcontent may be increased to a range above that normally feasibleheretofore, without producing a product of which the brittleness is sogreat as to preclude any subsequent machining operations.

Another object of the present invention resides in the provision of anovel iron-aluminum alloy containing, by Weight, approximately 18 to 31%of aluminum of which the brittleness is relatively low and which permitsof subsequent hot or cold working operations.

A further object of the present invention resides in the provision ofFe-Al alloys in which the brittleness is controlled to a degree notrealizable heretofore.

A further object of the present invention resides in the provision of aprocess for the manufacture of iron-aluminum alloys in which the thermalstresses are reduced and incipient boundary separations are controlledto fall within acceptable values.

A still further object of the present invention resides in a novel alloyprincipally containing iron and aluminum and having a relatively highproportion of aluminum which has magnetic properties and may be producedin the form of thin sheeting or foil.

Another object of the present invention resides in the provision of aprocess for the manuafcture of an ironaluminum alloy which permits theobtaining of very 3,386,819 Patented June 4, 1968 small thicknesses,accurate dimensions, and cold working treatments as well as subsequentheat treatments.

A further object of the present invention resides in the provision of aprocess for producing a low density ironbase magnetic alloy and in theresulting product which not only exhibits such low density properties,but also an oxidation resistance that is considerably higher than thatof other iron-aluminum alloys as well as stainless steel.

Still another object of the present invention resides in the provisionof a process for producing an iron-alu-minum alloy having neutronabsorption properties that are distinctly lower than those of stainlesssteel and having a yield strength that is considerably higher than thatof stainless steel.

Still a further object of the present invention resides in the provisionof a process for producing an iron-aluminum alloy and the alloyresulting from such process which maybe used in nuclear reactors and hassuch properties and characteristics as to obviate the need for enrichedfuels.

The present invention will be more clearly understood from a perusal ofthe following description of a number of examples of practicalapplication of the process in ac cordance with the present invention forthe preparation of an iron-aluminum alloy, the said examples being givenonl for illustrative purposes and without implying any limitation on thepresent invention. The single figure of the accompanying drawing shows adiagram illustrating the corrosion of an alloy in accordance with US.Patent 3,303,561, the patent application, in a carbon dioxide gasatmosphere as compared to that of a stainless steel under similarconditions.

Examples 1 through VI illustrate the basic process of US. Patent3,303,561 and are included herein for a comlete disclosure so as toprovide a full understanding of the present invention.

EXAMPLE I The alloy to be produced has the following composition:

Electrolytic iron kgs 3 Aluminum of 99.99% purity kg 1 Zirconium grams 4(a) Melting and casting.The 3 kilograms of electrolytic iron are meltedand brought to a temperature of 1,600 C. in a vacuum of the order of 10millimeters Hg; aluminum of 99.99% purity is then added thereto,followed by zirconium; the temperature is reduced to 1,450 C. and themolten mixture is poured off in vacuo, again of about 10" mm. Hg, intoan ingot-mold which has been heated to 620 C.

Finally, the cooling rate is limited to approximately 50 C. per hour. Itshould be noted in passing that preheating is of course necessary inthis example only on account of the fact that the casting mass employedin this example is small.

(b) Roughing-downr-The ingot which is obtained from Step (a) above aftercooling is fitted with a metallic jacket, for example, of ordinary steel(XC 12 or XC 35 in particular). The covering of the ingot may be carriedinto effect by means of any one of the methods of conventional cladding,for example, by welding a sheet which has previously been wrapped aroundthe ingot, by

cold-state hydrostatic cladding, etc. The thickness of the jacket isobviously designed so that the subsequent mechanical treatments permit athickness to remain which is such that there is no danger of tearing.This thickness was of the order of 2 mm. in the example described.

The composite work-piece formed by the ingot which is covered with itsjacket is subjected to a series of rolling passes at 1050 C., each passnecessarily resulting in a reduction in thickness which is sufiicient towork-harden the metal right through.

The presence of the jacket makes it possible to facilitate the surfaceflow of the alloy and permits the presence of deformations which theingot would not withstand if it were treated in the uncovered state.

In the example referred-t0, each pass resulted in a reduction inthickness of 2 mm., while between two successive passes, there wascarried out a reheating for a period of two minutes, thereby bringingthe temperature back to 1050 C. The thickness of the compositework-piece can thus be reduced without difficulty to approximately 2 mm.It is apparent that the reheating treatment is only necessary on accountof the fact that the temperature of the work-piece falls substantiallyas a result of the small dimensions of the latter.

The composite work-piece can then be freed of its steel jacket (thethickness of which has obviously been substantially reduced to the sameextent as those of the work-piece) by different methods. The jacket,which in the example described only remains in the form of a film of theorder of a few tenths of a millimeter, can be, for example:

Detached by machine-cutting of the jacket along one of the sides of thecasing,

Destroyed by chemically dissolving the jacket in a mixture of 50% nitricacid and 50% water (the iron-aluminum alloy having good resistance toattack by dilute nitric acid),

Destroyed by selective oxidation of the jacket by air heating or heatingin an oxidizing atmosphere.

(c) Cold wrking.The alloy which is thus obtained may be subjected tosubsequent mechanical operations which result in limited deformations,for example, deformation by rolling at room temperature with annealingtreatments between successive rolling passes.

EXAMPLE II (a) Melting and casting.A cast is prepared under conditionswhich are similar to those of Example I starting with 2.9 kilograms ofelectrolytic iron, 1.1 kilograms of aluminum and 4 grams of zirconium.The temperature is then raised to a few tenths of degrees above thesolidification temperature (or liquidus temperature) of the alloy andthe latter is poured off in vacuo into a preheated ingotmold. Thecooling process is then carried out as in Example I. The alloy which isthus cast has the following composition by weight:

Percent Iron 72.2

Aluminum 27.7

Zirconium 0.1

In addition thereto, analysis shows traces of carbon, nitrogen,phosphorus and sulphur in the following proportions:

Percent Carbon 0.01 Nitrogen 0.01 Phosphorus 0.002 Sulphur 0.002

(b) R0ughing-d0wn.-The ingot thus produced is capable of undergoinglathe turning work when use is made of tools having great hardness suchas tungsten carbide tools. The quality of machining is improved bymaintaining the alloy at 400 C. during the machining operation.

The said machining operation may not be necessary in the case of certainsurface conditions and when the roughing-down operation consists in arolling process which can be performed after cladding according to aprocedure which is similar to that described in the previous example.However, such a machining operation is necessary for the purpose ofshaping the ingot when the treatment involves extrusion of the jacketedingot.

When the extrusion process is intended to result in a full rod or slug,the lathe turning operation is per-formed with a view to obtaining acylinder having a rounded front end. The work-piece which is thusmachined is covered by means of any conventional process with a steeljacket having a shape which is adapted to that of the said work-pieceand a thickness of a few millimeters. It may be useful to replace mildsteel by other metals or alloys such as iron-aluminum alloys containinga low percentage of aluminum, which have the advantage of betteroxidation resistance and, in certain cases, nickel or eupronickel.

The composite part which is thus obtained is then pressextruded at 950C. At this temperature, it is possible to reach an extrusion ratio ofthe order of 1:30, or in other words, it is possible to prepare rods of11 mm. diameter from machined ingots of 60 mm. diameter.

A similar process makes it possible to obtain tubes having a thicknesswhich is less than one millimeter. In this case, the lathe turningoperation is performed with a view to producing a hollow cylinder whichis then clad both internally and externally.

After extrusion, the separation of the alloy and its steel jacket can becarried out in accordance with any one of the processes which havealready been referred to in Example I, for example, by chemicaldissolution in a solution composed of 50% 'Water and 50% nitric acidwhich rapidly dissolves the jacket, by oxidation of the jacket, by airheating or in an oxidizing atmosphere. In the latter case, the jacketdisappears whereas the alloy is not attacked by virtue of its highresistance to oxidation.

(0) Cold worlcing.-The extruded product obtained may, in certain cases,be employed as it stands, inasmuch as it has a good surface to finish.However, it may, if necessary, be subjected to a further cold workingtreatment and may, for example, be threaded on a threadcutting lathe. Infact, the grain size after extrusion is reduced to 20 or 30 microns andaccordingly permits machining thereof.

The part which has been either machined or extruded may be subjected toa heat treatment for a period of one hour at 800 C.; after this heattreatment, the extruded product has the following characteristics:

EXAMPLE III The same operations as in Example II (melting, casting,machining, jacketing, extrusion and elimination of the jacket) have alsobeen applied to an alloy containing 25% aluminum by weight, thecomposition of which is as follows:

Percent Iron 74.9 Aluminum 25.0 Zirconium 0.1

and which shows traces of impurities in the following proportions:

Percent Carbon 0.01 Nitrogen 0.01 Phosphorus 0.002 Sulphur 0.002

After heat treatment at 800 C., the product obtained has thecharacteristics which are given in the table below:

Ultimate Elonga- 1 Brittle fracture.

The product which is obtained as a result of extrusion may be subjectedto further processing so as to permit a final cold rolling in lightpasses. This processing will consist (if necessary after burnishing ofthe extruded product), for example, of a further jacketing operation,followed by a rolling operation between 500 and 600 C. for the purposeof orienting the crystals. The product thus obtained, still in thejacketed state, can be cold rolled.

The single figure of the accompanying drawing represents the oxidation(expressed as increase in weight per unit area), against time (expressedin hours) of two materials in a carbon dioxide atmosphere at 700 C.under a pressure of 60 kg./cm. The curve I corresponds to theiron-aluminum alloy in accordance with Example III herein. Curve IIcorresponds to an 18-12 niobiumstabilized stainless steel which is knownfor its good resistance to corrosion by carbon dioxide gas at hightemperature. It may be readily seen from this figure that, at the end ofa period of exposure of 5,500 hours, the corrosion of the iron-aluminumalloy is less than one half that of stainless steel.

EXAMPLE IV (a) Melting and casting.An alloy containing 79.6% iron, 17.2%aluminum and 2.8% beryllium is prepared from the following constituents:

Iron kgs 3 Aluminum kg. 0.650 Beryllium kg 0.105 Zirconium -grams..- 15

The melting and casting operations are carried out as in the previousexamples; in the as cast state, this alloy has the following properties:

Grain size, approximately 0.15 mm. Brinell hardness, A=320.

(b) Roughing-dowm-The ingot is rolled at a temperature of 1050 C. inpasses which each result in a reduction in thickness of 1 mm. to a finalthickness of 2 mm.; in this state, the alloy has a Brinell hardness13:330.

Thereafter, a heat treatment at 1,100 C. makes it possible to reduce theBrinell hardness number to 260.

EXAMPLE v (a) Melting and casting.The same operations of melting andcasting are applied to an alloy containing 25% aluminum having thefollowing composition by weight:

(b) Roughing-down-hot-state deformation. Rolling passes are effected at1,050 C. and a reheating treatment is performed between each rollingpass for a period of two minutes.

A reduction value of less than can be obtained with final thicknesses ofthe order of one millimeter.

(c) Cold-state def0rmati0n.-After hot rolling, the rolling treatment canbe repeated at room temperature.

It is accordingly possible as a result of cold rolling to obtainreduction values of 50%.

After rolling, the Vickers hardness number of the product is 500 HV;heat treatments by annealing at 950 C. make it possible to reduce thishardness number to 280 HV.

and impurities of the same order as in Examples II and III.

The product obtained has the following characteristics:

Ultimate Yield Elongation Temperature, C. tensile strength, at rupture,

strength, kgs./mm. percent kgsJmrn.

l Brittle fracture. 2 Ductile fracture.

The examples which are given above, although obviously not limitative,show that the process in accordance with US. Patent 3,303,561 makes itpossible to obtain iron-aluminum alloys in which the proportion ofaluminum substantially exceeds the values of 16 to 18% by weight whichwere hitherto accepted as the limit starting from which alloys no longerhad mechanical properties which permitted their subsequent working. Theironaluminum alloy which may now be produced by this process makes itpossible to approach the limit of solubility of aluminum in iron (34%approximately) while retaining good mechanical properties. If suchmechanical properties are not essential requirements, a smallprecipitation of aluminum is permissive at the expense of a verysubstantial reduction of mechanical characteristics, thereby making itpossible to reach a proportion of approxi mately 40%.

The examples described below illustrate embodiments of the presentinvention, in which the alloy includes additives selected from the groupconsisting of yttrium and rare earths in proportions such that the totalpercentage of said additives contained in the solidified alloy is withinthe range of 0.4 to 4%.

When the alloy is for use in the nuclear. field, the addition element ischosen among those which have the smallest neutron-capturecross-section; yttrium will usually be adopted. In other cases, and forpurposes of economy, mixtures of rare earths can be employed of the typeknown as Mischmetal.

The invention also provides improved iron-aluminum alloys obtained as aresult of the application of the process referred-to above, the saidalloys containing addition elements selected from the group consistingof yttrium and rare earth metals in a proportion which ranges from 0.4to 4% by weight, preferably at least 1% to 4% by weight, and havinggrain dimensions smaller than 20 after mechanical working.

In'the preferred case of alloys which only contain iron, aluminum andsuch additives, the aluminum content is advantageously comprised between18 and 31%, this lastmentioned value corresponding to the appearance ofa precipitate of the phase Fe-Al The incorporation of the additive oradditives is carried out during the melting process, preferably byintroducing it or them in the bath of molten iron. During the next phase(up to solidification of the ingot), losses are liable to occur; inorder that the final content should comprise between 0.4 and 4% byweight, it may therefore be found necessary to introduce into the moltenmass an addition which would correspond to a higher percentage contentthan that which is sought.

The casting and solidification processes being conducted underconditions which are comparable with those described in Examples I-VI, asubstantial reduction in the mean grain size of the ingot is noted, withthe advantages thus obtained. In the as-cast state under the sameconditions of production, the grain size is, for example, reduced by afactor of the order of with an addition of 1% of lanthanum or yttrium.The result thereby achieved is a reduction in brittleness and animprovement in machinability of as-cast ingots; lathe turning can beaccomplished under the usual conditions of temperature and in additionyields products of a quality which is at least equal to that ofmachining at 400 C. which is described in Example II.

This reduction in grain size is retained at the time of roughing-down,in particular when carried out by the hot extrusion process aftercladding as described above. it is possible in particular to obtaingrains having a size of the order of 10 microns after extrusion at 950C. and, in a general manner, the grain size remains smaller thanmicrons.

An attempt can be made to explain the achievement of a reduction ratherthan an increase in grain size, although it will be understood that theexplanation which now follows is given only by way of indication andcannot be considered as lying at the heart of the present invention. Theaddition of rare earths in proportions higher than 0.4% is accompaniedin the as-cast state by the precipitation at the grain boundaries of asecond phase which is totally different from the main phase which formsthe matrix (Fe-Al phase up to 31% by weight of aluminum). At the time ofextrusion, these precipitates are aligned in the direction of extrusionin fibers having a density which increases with the percentage of rareearths contained in the alloy. The increase in grain size is accordinglyprevented by this precipitation.

1n the case in which lanthanum is employed as an additive, theprecipitates observed are Fe-Al-La compounds. This precipitate isbrittle, but the mechanical properties of the alloy are neverthelessimproved. It is possible that the beneficial effect of lanthanum in thisparticular case not only lies in its grain-refining action but also inthe fact that the matrix itself becomes more ductile as a result of thepurification which is due to the precipitate or to the lanthanum.

The effect of reduction in grain size is maintained in the case ofannealing operations, even at high temperature. Thus, alevel-temperature stage of one hour at 1l50 does not result in anyperceptible increase in grain size since the grains are stabilized bythe precipitates.

This stabilization constitutes an appreciable advantage which is ofspecial value for welding purposes. It is in fact possible to weldiron-aluminum alloys having a high aluminum content (for example 40% interms of atomic ratio) by conventional methods such as by arc welding inan argon atmosphere. However, the welded zone has a casting structurewhich is much more brittle than that of the base alloy. In order toprevent the alloy from melting, diffusion welding can be performed inthe solid state but, in order to break up the oxide layer and ensure agood weld, the parts to be assembled have to be heated to at least 1100C. for a period of a few minutes; this treatment produces a substantialincrease in grain size within the alloy according to the above examples,whereas the alloy which contains 0.4 to 4% of rare earths retains itsfine grain structure and good properties which result therefrom.

Finally, the alloy which is prepared in accordance with the presentinvention retains improved machinability after extrusion as well asenhanced mechanical properties.

EXAMPLE VII This example refers to a lanthanum alloy which exhibitsafter roughing-down an elongation of 11% under tension at normal speedand at room temperature.

The alloy to be produced has a composition which is comparable with thatwhich is given as Example I, but lanthanum is added. The melt isprepared from:

Electrolytic iron kg. 3 Aluminum of 99.99% purity kg. (24% 0.960Lanthanum gr. 1% 40 (a) Melting and casting.The conditions of meltingand casting are similar to those of the process described hereinabove,i.e, the iron is melted and brought up to 1600 C. in vacuo, aluminum isadded, lanthanum is added at the same time as aluminum, the temperatureis reduced to 1450 C. and the casting operation is performed in aningot-mold which has been pre-heated to 620 C. Finally, the cooling rateis limited to approximate ly 50 C. per hour.

The conditions of casting in vacuo as adopted in the case which iscontemplated resulted in a loss of lanthanum and analysis of the ingotrevealed only 0.7% by weight of lanthanum in addition to the usualtraces of carbon, nitrogen, phosphorus and sulphur.

(b) Roughing-down.--The roughing-down process can consist of a series ofoperations which are similar to those described in Example II.

The ingot can subsequently be machined on a lathe by using tools of highhardness. The machined workpiece is clad with a steel jacket a fewmillimeters in thickness. The composite workpiece obtained ispress-extruded and the steel jacket is removed, for example by chemicaldissolving in a solution composed of 50% water and 50% nitric acid.

As has been indicated above in a general manner, it is no longernecessary to maintain the alloy at 400v C. during the machiningoperation.

(0) Cold working.-The extruded product obtained has a better state ofsurface than that of the product according to Example II. Moreover, itcan be cold-worked and this operation (which is already possible withthe alloy in accordance with said Example II) is further facilitated bythe smaller grain size which is in the vicinity of 10 microns afterextrusion.

The machined workpiece can undergo a heat treatment for a period of onehour at 1150 C. without any appreciable increase in grain size.

After treatment for a period of one hour at 800 C. followed by slowcooling (30 C. per hour), the extruded product has the followingcharacteristics:

Elongation Ultimate Yield at rupture Temperature, C. tensile strength,(Normal strength, kg./mm. deformation kg./mrn. rate),

percent The above example shows that the invention makes it possible toimprove those iron-aluminum alloys in which the proportion of aluminumexceeds 18% by weight to an extent which greatly facilitates machining.reduces the wear of tools and lowers the price and duration of themachining operation.

9 EXAMPLE VIII The iron and aluminum contents of the alloy produced arethe same as in Example I, but the final percentage of lanthanum is 3.75%by weight. The successive steps are the same. The extruded product hasthe following characteristics at 20 C.:

Ultimate tensile strength kg./m1n. 66 Yield strength kg./mm. 58Elongation at fracture percent 1.0

The ductility of the alloy is much decreased by the increase inlanthanum content; on the contrary the elastic limit is increased.

EXAMPLE IX The iron and aluminum contents are again the same as inExample I, but lanthanum is replaced with yttrium, the final proportionof which is 0.4%. The successive process steps are the same as above.The product exhibits the following characteristics:

The composition and process steps are the same as in Example IX, excepta regards the yttrium content, which is increased to 3.15%. The producthas the following characteristics at 20 C.:

Ultimate tensile strength kg./mm. 80 Yield strength kg./mm. 55Elongation at fracture percent 2 It is wholly apparent that the scope ofthe present invention extends not only to the process which has justbeen described and to all alternative forms thereof which are within thescope of equivalency thereof, but also, by way of new industrialproducts, to the alloys which are obtained as a result of theapplication of the process in accordance with the present invention.

Thus, while we have described several specific examples in accordancewith the present invention, it is obvious that the same is not limitedthereto, but is susceptible of numerous changes and modifications withinthe scope of a person skilled in the art, and we therefore do not wishto be limited to the details described herein, but intend to cover allsuch changes and modifications as are encompassed by the scope of theappended claims.

We claim:

1. An alloy principally composed of iron and aluminum and of a smallamount of at least one additive selected from the group consisting ofzirconium, niobium, titanium, yttrium, the rare earths, boron andmixtures thereof in which the aluminum content is greater than about 16%by weight of the alloy and the iron content is less than about 84% byweight of the alloy while the additive content is less than one-tenththe content, by weight, of the aluminum content but more than at least1% by weight of the ingot, said alloy being characterized by arelatively low brittleness permitting machining operations.

2. A process for the preparation of an iron-aluminum alloy comprising:melting an amount of iron which corresponds to a proportion less than84% by weight of the alloy, adding to the molten iron the otherconstituents of the alloy, casting the melt at a temperature slightlyabove the solidification point of the alloy, cooling the alloy tosolidify it in the form of an ingot, and subjecting the ingot tohot-state mechanical working and deformation to destroy the castingstructure, including the incorporation with the melt of at least oneadditive selected from the group consisting of yttrium and rare earthsin proportions such that the total percentage of said additivescontained in the solidified alloy is within the range of at least 1% to4% by weight.

3. A process according to claim 2, wherein the percentage content ofiron ranges from 69% to 82% by weight and the percentage content ofaluminum ranges from 18% to 31% by weight.

4. A binary iron-aluminum alloy in which the aluminum content is greaterthan 18% by weight containing a proportion ranging from at least 1% to4% by weight of additives selected from the group consisting of yttriumand the rare earths and having grain sizes smaller than 20p. aftermechanical working and deformation in the hot state.

References Cited UNITED STATES PATENTS 2,768,915 10/1956 Nachman et al.l24 X 2,804,387 8/1957 Morgan et al. 75l24 2,846,494 8/ 1958 Lindenblad75l24 X 2,859,143 9/1958 Nachman et a1 148-2 3,026,197 3/1962 Schramm75l24 3,144,330 8/1964 Storchheim 75l24 X 3,303,561 2/1967 Cabane et a1.29528 JOHN F. CAMPBELL, Primary Examiner.

PAUL M. COHEN, Assistant Examiner.

